The present invention relates to an active matrix display device using current-dependent light-emitting elements as pixels at cross points in the matrix form, and in particular, to a current-dependent light-emitting element drive circuit used at each of cross points.
In a conventional active matrix display device, a plurality of first lines or scanning lines extend in parallel with one another and a plurality of second lines or data lines extend perpendicular to the first lines to form a plurality of cross points arranged in a matrix form. A current-dependent light-emitting element drive circuit is connected to one of the first lines and one of the second lines at each of the cross points to form one of the pixels in the display device. The current-dependent light-emitting element drive circuit comprises the current-dependent light-emitting element to be connected to a current source. A current control transistor is coupled to the first and the second lines and is connected in series with the current-dependent light-emltting element. The current control transistor controls current flowing through the current-dependent light-emitting element from the current source in response to selection signals selectively applied to the first and second lines. The current-dependent light-emitting element emits light with an intensity dependent on the current controlled.
As the current-dependent light-emitting elements, organic and inorganic EL (electroluminescence) elements, and LEDs (light-emitting diodes) are used and their luminance is dependent on or controlled by the current flowing in the element.
The display device has been widely used in televisions, portable terminals and the like, wherein the character display is performed on the dot matrix by arranging the light-emitting elements in a matrix array.
It is advantageous that the display does not require the backlighting as opposed to the liquid-crystal display devices, and is large in the angle of visibility.
The display device of the active matrix type performs the static drive by combination of the transistors and the light-emltting elements and is capable of providing high luminance, high contrast, high accuracy and the like as compared with the passive matrix type display which performs the dynamic drive.
Conventional display devices of the active matrix type are disclosed in JP-A-2 148687 and in a paper entitled "DESIGN OF A PROTOTYPE ACTIVE MATRIX CdSe TFT ADDRESSED EL DISPLAY" by J. Vanfleteren et al, Eurodisplay '90, Society for Information Display, pp. 216-219.
However, in the conventional active matrix display device, a transistor is connected to the light emitting element in series and controls the current flowing therethrough. Therefore, the light intensity or luminance of the light-emitting element is also changed in dependence on variation of properties of the transistors. This results in impossibility of correct control of the light intensity emitted.
Further, when the light-emitting element in repeatedly driven at a high frequency by repeatedly scanning the scanning lines in the display device, a user is caused by flickering to be tired to watch the display.